Co-owned United States patent application publication number US 2003/0106993 by Chen et al., which is hereby incorporated by reference herein in its entirety, discloses use of nuclear electromagnetic irradiation for density determination. In this, knowing the density of a sample of interest, for example, a fluid in a borehole, can provide important information to end users. For example, dead crude oil (i.e., oil at the formation surface and at ambient pressure) consists primarily of carbon and hydrogen with some contaminants or unwanted elements such as sulfur which constitute by weight a few percent of the oil. Generally the economic value of the crude oil increases with its hydrogen content compared to its carbon content. Asphaltenes are primarily large, aromatic molecules of considerable densities. Thus, in crude oil, a high density is generally indicative of high asphaltene content. The presence of a large amount of asphaltenes in oil is undesirable from both a production viewpoint and from a processing perspective. In production, asphaltenes are known to plug oil wells.
The measurement of fluid density, in conditions encountered downhole, is a service that is desired in the oilfield exploration and development industry. A nuclear based measurement is one reliable and rugged way to obtain the density from a large variety of fluids. Optical techniques currently used for the analysis of downhole fluid samples are not sufficient to completely characterize the downhole fluids drawn into sampling devices. Mixtures of several different liquids are likely to be found in formation fluids causing difficulties in optical measurements. Two fluids may have the same density but different optical absorption characteristics. One example is a heavy oil precipitating asphaltene from solution, the average density of the fluid is unchanged as the precipitate forms, but the optical properties of the fluid are dramatically affected.
One technique for measuring fluid density that overcomes this limitation of optical techniques is a photon attenuation measurement, in which the attenuation of gamma ray photons passing transversely through a pipe containing a fluid is measured. However, this technique is intended for use in locations where spatial constraints are not too severe. In deep boreholes of relatively small diameter, and in certain borehole tools, the space available for the apparatus required for such a measurement can be limited.
In addition to the failure to provide for a small space measurement, previous attempts to measure density downhole have utilized a chemical radiation source that carries high liability and operating requirements. Using a chemical radiation source introduces other concerns as well. Due to the danger associated with these sources, it is necessary to use a source with a low intensity to meet stringent regulations. This low intensity source requires both longer measurement times and a larger flow line that may not fit in the constraints down hole.